The invention relates to a method of manufacturing a colour television display tube having an envelope comprising a conical portion and a window portion sealed together in a vacuum-tight manner by means of a sealing glass. Prior to sealing said portions together, a gettering device is provided in a place situated inside the envelope of the tube. The gettering device comprises a source of evaporable gettering metal and a pulverulent material gas source for releasing gas upon heating. After evacuation of the display tube the gas is released from the gas source and the gettering metal is evaporated.
The invention furthermore relates to a colour television display tube thus manufactured, as well as to a gettering device suitable for use in the above-mentioned method.
A method of the kind described above is disclosed in British Patent Specification No. 1,226,728.
The source from which the gettering metal is evaporated usually consists of a mixture of nickel powder or a nickel alloy powder and a powdered alloy of gettering metal and aluminium. Suitable gettering metals are barium, strontium, calcium and magnesium. A frequently used source of gettering metal consists of a mixture of nickel powder and barium aluminium (BaAl.sub.4) powder, which mixture contains approximately 40-60% by weight of nickel powder.
The source from which the gas is released as a scattering medium for the evaporating gettering metal usually consists of a nitrogen compound or a hydrogen compound from which the nitrogen or hydrogen is released by thermal decomposition. The quantity of released gas is adsorbed by the evaporated gettering metal. Examples of these compounds are iron nitride, nickel nitride, barium nitride, germanium nitride, titanium hydride and barium hydride. A very suitable gas source consists of pulverulent iron nitride (Fe.sub.4 N).
The usefulness of a gettering device is determined to a considerable extent by the extent to which it can withstand the influence of the surrounding atmosphere. The chemical composition of the components of the gettering device should not change under the conditions prevailing during the storage of the gettering device or during the manufacture of the tubes in which they are used. In this respect problems occur in particular when--as described in British Patent Specification No. 1,226,728--the gettering device is mounted in the tube before the display window of the tube has been sealed to the cone of the tube by means of a sealing glass. These envelope parts are sealed together in a furnace at a temperature of approximately 450.degree. C. The sealing of the envelope parts lasts approximately one hour, and the components of the gettering device as such cannot withstand the influence of the surrounding atmosphere at the temperature occurring during this sealing process. British Patent Specification No. 1,226,728 proposes the use of a protective foil or layer of, for example, aluminium over the surface of the gettering device exposed to the atmosphere. However, this measure has proved to be not quite satisfactory in the conditions prevailing during the sealing together of window and cone of the tube.
Regarding the source of gettering metal, it has been suggested to replace the nickel powder in the source of gettering metal by a nickel-titanium compound or an iron-titanium compound which is more chemically resistant to the atmosphere existing during the sealing process. For a source of gettering metal consisting of a mixture of barium-aluminium powder (BaAl.sub.4) and nickel powder, U.S. Pat. No. 4,077,899, the contents of which are incorporated herein by reference, discloses a way to improve the chemical resistance of the the mixture. This improvement is brought about by ensuring that the nickel powder has an average grain size smaller than 80 microns and a specific surface area smaller than 0.15 m.sup.2 per gram, while the average grain size of the barium-aluminium powder is smaller than 125 microns.
It has proved necessary when using a gettering device which comprises a gas-releasing material, to take measures which prevent attack of the gas-releasing material by the ambient atmosphere at a premature stage of the manufacture of the tube in such a manner that it is unfit for use when the gettering device is fired.